Herbicide Resistant Pigweed 3: Management in Horticultural Crops

This video covers the best management practices for Palmer Amaranth and waterhemp in horticultural cropping systems.
Herbicide Resistant Pigweed 3: Management in Horticultural Crops - Videos

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- [Narrator] Herbicide resistant Palmer Amaranth and waterhemp are a serious growing program for Pennsylania agriculture, including vegetable and fruit crops.

They're currently present in at least 17 Pennsylvania counties and are continuing to spread to new fields.

They're resistant to multiple herbicide modes of action and we advise that all Palmer and waterhemp in Pennsylvania are likely resistant to both Glyphosate and ALS herbicides.

Other things that make them difficult to control are that they germinate throughout the season, they grow very quickly, and they're highly drought tolerant.

Effective herbicides only manage seedlings under six inches tall.

In vegetable and fruit crops, herbicide options are especially minimal.

Because of this, suppression of these weeds requires diligent scouting, prevention, and diverse management tactics.

This Learn Now video will outline the best management practices for herbicide resistant Palmer and waterhemp for horticultural crops.

The first best management practice is to monitor any inputs coming onto the farm particularly seeds, to make sure any seeds entering the farm are certified as clean.

Avoid buying seeds from locations that may have Palmer Amaranth or waterhemp on the site.

In Pennsylvania, laws are in place to prevent the contamination of crop seed.

Be cautious when bringing in specialty seed mixes, such as wild life feed and native seed mixes as they could be a potential carrier of pigweed seeds, as well.

Additionally, avoid spreading manure sourced from livestock that may have been in contact with Palmer or waterhemp as pigweed seed survive digestion and end up in manure germinating in the field after its spread.

Secondly, utilize diverse weed management practices in addition to using chemical weed control.

Because relying solely on herbicides for pigweeds in vegetable and fruit systems will rarely provide complete control.

Herbicide options for herbicide resistant pigweeds are very limited and the options that do exist will not provide full season control with pigweed.

This is a problem because Palmer and waterhemp seeds keep emerging throughout the season.

Additional control methods for problem weeds include mechanical weed removal, such as mowing and cultivation.

Also, cultural control practices like plasticulture and in some cases, cover cropping can be used to effectively block out weeds.

For more information on plasticulture and cover cropping in horticultural crops can be found on the Penn State University extension website.

When herbicides are needed to control Palmer and waterhemp, here is a list of soil applied pre-herbicides, and folia applied post-herbicides that work well on Palmer, as well as some that do not provide control.

You'll see this table again later in the video.

Third, Palmer Amaranth and waterhemp become much more difficult to control as they grow larger because herbicide application is largely ineffective on plants over six inches tall.

We highly recommend cultivating before the plants reach three inches in height in combination with herbicide application.

Four, you can design crop rotations in a way to discourage pigweed growth and to help the crop be more competitive against weeds.

In Pennsylvania, pigweeds germinate starting in May and they start dropping their seed in August.

So planting early season crops that are established before Palmer Amaranth and waterhemp emerge in the spring means that the crop can develop a canopy and healthy root system before the weeds really step in.

Then, depending on the crop that early season crop may be able to be harvested earlier on before any remaining mature pigweed plants produce their seed in the fall.

Once the crop is harvested, remaining pigweeds can be more easily removed or mowed before they produce seed.

It's important to not let Palmer and waterhemp mature and drop seed as each plant can produce up to one million seeds.

In fact, once resistant pigweed are already established in a field, the most important thing to do is get rid of that seed bank and make sure that no more seeds are allowed to fall in the field.

If any Palmer or waterhemp plants survive early season control, it's really critical to physically remove them by hand, or with a hoe.

Even one plant left in a field can produce up to a million seeds.

Since pulled plants can re-root transport them out of the field to the field edge and then bury or burn them there.

Although removing weeds can be time-consuming depending on the number of plants, it really is worth the time and energy so that a large infestation doesn't develop next year.

Six, if a Palmer or waterhemp infestation survives initial control and is over six inches tall further herbicide application will likely be ineffective.

If the infestation is too severe to take out all the plants with hand pulling, the next option is to mow or disc the weeds.

Of course, the availability and timing of this tactic depends on the cropping system.

In vegetable crops, mowing can and should be done after harvest to stop Palmer from going to seed.

Mowing is a more readily available option in cover cropped perennials like orchards and vineyards or in vegetables with cover cropped aisles.

We've seen cases where a pigweed infestation was so severe that it dominated the field and the field had to be mowed down, sacrificing the crop.

Hopefully this never happens to you but if you do have to mow down pigweed an important thing to remember is that Palmer and waterhemp often regrow after mowing.

So repeated mowing will likely be necessary for total control.

When possible, when decreasing crop row spacing can decrease weed germination by reducing light to the seeds and small weed seedlings.

Planting crops with canopies that develop quickly can help achieve this too.

This isn't always possible in all fruit and vegetable crops but when it is feasible, this can help optimize weed control by blocking out light to the weeds.

Eight, controlling remaining Palmer and waterhemp plants after harvest is complete is essential to stopping those plants from producing weed seed and halting new weed emergence.

This can be done by a combination of herbicide burndown, mowing and cultivation depending on what's most appropriate for this specific crop, cropping system, and the specific farm.

I mentioned earlier in the video that herbicide options are very limited for controlling Palmer and waterhemp in horticultural crops.

Even though options are limited, a burndown herbicide application after harvest can do a lot for post-harvest control.

Herbicides will only be effective on small pigweed plants and the product depends on the crop that will be planted in the field the next year.

Other control tactics will still be necessary especially if the plants are taller than a few inches.

Here is some suggestions for specific crops.

Please pause the video here, if you'd like extra time to look at these before moving on.

As I mentioned at the beginning of the presentation the herbicide resistance that Palmer Amaranth and waterhemp has is primarily Glyphosate and ALS resistance in Pennsylvania.

They are resistant to other modes of action and those populations are located in other states and could spread to Pennsylvania.

It's important to note that we don't currently know if any herbicide modes of action that are being developed.

Even if new modes of action are developed Palmer and waterhemp have the ability to develop new resistance to new modes of action that they're exposed to.

This highlights our need to use the herbicides we have judiciously and to integrate alternative tactics to optimize weed control.

Palmer Amaranth and waterhemp may be in a field near you.

So developing a proactive management plan now will save time and money later.

For further information, or to report a possible infestation talk to your county extension office.

For further resources, visit our website.

Also be sure to watch our other Learn Now videos on Herbicide Resistant Pigweed including Weed ID, Proper Prevention and Disposal Practices, and Management and Conventional Row Crops in Organic Systems.

Thanks for watching.


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